Scrape an edge as Magnesium will be very shiny. Then take the scraping and drop in Vinegar and it should outgas. Quick check....
•Try the vinegar test. Drop a little bit of 5% acidity white vinegar on the back of the part. If the vinegar bubbles, the part is magnesium.
•Scrape some of the finish with a razor blade. If the vinegar test seems insubstantial (it isn't, but some people like to be triple sure), scrape some of the finish off the back of the rim. Magnesium will appear silver in color. The cuttings or chips will be dusty or flaky. By comparison, aluminum will show a whiter color when scraped and the chips will be gummy or soft